Oxide varistor

ABSTRACT

An oxide varistor is prepared from a basic composition formed of 70 to 14 mol% of at least one MeIO2, 29 to 85 mol% of ZnO and 1 to 20 mol% of Sb2O3, 1 to 20% by weight of Bi2O3, and 0.5 to 10% by weight of at least one MeII2O3 based on said basic composition, MeI being selected from Ti, Sn and Zr, and MeII being selected from Fe, Cr, Mn and Co.

United States Patent Ichinose et al.

1451 Aug. 12, 1975 OXIDE VARISTOR [75] Inventors: Noboru lchinose; Yuhji Yokomizo,

both of Tokyo, Japan [73] Assignee: Tokyo Shibaura Electric Co., Ltd., Kawasaki. Japan [22] Filed: Sept. 11, 1973 [21] Appl. N0.: 396,135

[30] Foreign Application Priority Data Sept. ll, 1972 Japan 47-90446 Sept. 11, 1972 Japan 47-90447 Sept. 18, 1972 Japan 47-92897 [52] US. Cl 252/520; 252/518 [51] Int. Cl. H01B l/08 [58] Field of Search 252/520, 518

[56] References Cited UNITED STATES PATENTS 3,669,907 6/1972 Kohashi et al. 252/520 X 3,778,743 12/1973 Matsuoka et al. 252/518 X 3,805,114 4/1974 Matsuoka et al. 252/518 X 3,806,765 4/1974 Matsuoka et a]. 252/518 X Primary Examiner-Leland A. Sebastian Attorney, Agent, or FirmStewart and Kolasch, Ltd.

[ 5 7 ABSTRACT 5 Claims, 14 Drawing Figures Bi203 (wr PATENTEU AIJB12I975 3, 899,451

Bi203 (wT OXIDE VARISTOR This invention relates to a varistor prepared from an oxide semiconducttor, and more particularly to an oxide varistor having greater voltage-current nonlinear coefficient.

As one of circuit elements based on a semiconductor, a voltage nonlinear resistance element is known. SiC varistors are known as a typical example. The element of this kind has nonlinear voltage-current characteristics, namely, is sharply reduced in resistance with increasing voltage to permit electric current to be markedly increased accordingly and has consequently been widely used for absorbing abnormally high voltage or for stabilization of voltage.

With the recent marked advance in electronic art there is a growing demand for devices (IC, transistors, etc.) in which semiconductor are used. However, these electronic parts have disadvantage in common that they are weak against abnormal voltage and a settlement to this problem has been sought. Arresters, surge absorbers, etc., using the conventional SiC varistor are slow in charge response speed to impulses and incomplete for protection against surges involved in devices in which semiconductors are used.

With rotational devices, the art of breakers has been markedly developed. With the advent of a vacuum breaker, an on-off surge due to current suppression poses a problem. The protection against the on-off surge has heretofore been made by using a gap type arrester or a capacitor for electric power. In the gap type arrester, it is possible to absorb abnormal voltage due to a normal current suppression. However, there is still a problem in respsonding to impluses of the order of MHz in current unstable areas and impulses involved during re-ignition. When the capacitor is used, problems arise from the price consideration as well as from the setting of capacitance. Therefore, there is a demand for an inexpensive varistor element having excellent varistor characteristics.

Generally, the voltage-current characteristics of the varistor may be expressed approximately in the following equation:

I-(V/C) where current flowing through the varistor V voltage across the varistor C constant a nonlinear voltage coefficient Therefore, the characteristics of the varistor may be indicated by C and a or the two other constants which can replace them. Since accurate determination of C presents extreme difficulties, C is generally substituted by voltage Vc at a certain current C (mA). With the varistor voltage thus designated as Vc the voltage-current characteristics of the varistor may be indicated by V(' and the nonlinear coefficient a. An SiC varistor heretofore well known as a varistor element has as small a voltage nonlinear coefficient a as 37. A zener diode is known as having a larger voltage nonlinear coefficient. The zener diode is expensive and in addition has a limited usage voltage of 200 V at most and is consequently inconvenient for use with electronic devices requiring for high voltage. The zener diode involves a greater variation with temperature in the rising voltage and a smaller surge resistance, thus presenting practical problems.

US. Pat. No. 3,632,529 discloses a voltage variable resistor ceramic composition consisting essentially of zinc oxide and 0.05 to 10.0 mol of strontium oxide and, as an additive, 0.05 to 8.0 mol of bismuth oxide, lead oxide, calcium oxide or cobalt oxide. This ceramic composition is different from an oxide semiconductor composition of an oxide varistor according to this invention. The voltage nonlinear coefficient of the ceramic composition is of the order of 10 and is lower than that (more than 30) of the oxide varistor according to this invention.

US. Pat. No. 3,663,458 discloses a nonlinear bulktype resistor consisting essentially of a sintered body of 80.0 to 99.9 mol of ZnO, 0.05 to 10 mol Bi O and 0.05 to 10 mol in total of at least one member selected from the group consisting of C00, MnO In O Sb O TiO B 0 A1 0 SnO BaO, NiO, M00 Ta O Fe O and Cr O It will be evident from the following description that the composition of the sintered body is different from that of an oxide varistor according to this invention. According to this patent the content of SnO or TiO is as low as 0.05 to 10 mol and it is impossible to attain a predetermined performance in the practice of this invention. Furthermore, the resistor represents a relatively great change when electric current is supplied for a load life test.

An object of this invention is to provide an oxide varistor having a greater voltage nonlinear coefficient (a 30).

Another object of this invention is to provide a high performance oxide varistor capable of representing a lower rising voltage, a small change with temperature, a high surge resistance and a smaller change with time.

The other object will be apparent from the following description.

In accordance with this invention there is provided an oxide varistor prepared from a basic composition formed of to 14 mol of at least one Me'O 29 to mol of ZnO and l to 20 mol of Sb O l to 20 by weight of Bi O and 0.5 to 10 by weight of at least one Me" O based on said basic composition, wherein said Me' is a metal selected from the group consisting of Ti, Sn and Zr, and said Me" is a metal selected from the group consisting of Fe, Cr, Mn and Co.

This invention can be more fully understood from the following detailed description when taken in connection with reference to the accompanying drawings, in which:

FIG. 1 shows the content of Sb O and the resistivity of an Me'O -Zno-Sb O system in which the molar ratio of Me'O to ZnO is rendered constant;

FIG. 2 shows the molar ratio of Me'O to ZnO and the resistivity of the Me'O -Zno-Sb O system in which the content of sb og is rendered constant; and

FIGS. 3A to 6C show a relation between the content of Bi- O and the voltage nonlinear coefficient a of an Me'O -Zno-Sb O -Bi O -Me" O system in which Me"' 0;; is used as a parameter.

An oxide varistor according to this invention can be prepared in the following manners. Accurately weighed out raw material metal oxides having a predetermined composition ratio are mixed together at a ball mill, etc., and after preliminarily sintered at a relatively low temperature, for example, at 600 to 900C, are powdered by the ball mill. As raw material used in this case, use

may be made of metal compound, such as hydroxide, carbonate, oxalate, etc., which is convertible into oxide upon heating. The powder so obtained is mixed with a binder such as polyvinyl alcohol, etc. The mixture is shaped, under pressure of about I to 1,000 Kg/cm into a disc having a diameter of about 20 mm and a thickness of about 1 mm. The disc is sintered generally in an air atmosphere at a temperature of about l,()00 to l,300C. During the sintering it is kept at a maximum temperature for about I to 6 hours. Then, electrodes are baked to the resultant disc to obtain a varistor.

As mentioned above, oxide varistor according to this invention includes a basic composition formed of 70 to 14 mol of metal dioxide Me'O 29 to 85 mol of ZnO and l to 20 mol of Sb O The metal dioxide is selected from TiO SnO Zro or a mixture thereof.

The content of Sb O is l to 20 mol When the content (mol of Sb O of the basic composition i.e. Me O -ZnO-Sb O system is varied with the molar ratio of Me'O to ZnO fixed at 1:2, resistivity values as shown in FIG. 1 are obtained. As will be understood from this figure, when Sb O exceeds l mol the resistivity value is sufficiently lowered and is suitable for use as a varistor. However, when Sb O exceeds 20 mol then the resistivity value is increased and is unsuitable for use as a varistor. Even if the resistivity presents no problem, sintered bodies are so porous that no desirable varistor is obtained from the practical viewpoint. It is preferred that the content of Sb O be 5 to mol The contents of Me'O and ZnO are 70 to 14 mol and 29 to 85 mol respectively. When the molar ratio of Me'O to ZnO of the base Me'O -Zno-Sb O is varied with the content of Sb O fixed at 6 mol then the resistivity of the basic composition is as shown in FIG. 2. As will be appreciated from this figure, when the content of Me'O is outside the scope of 70 to 14 mol and the content of ZnO is outside the scope of 29 to 85 mol a resistivity is increased and the composition is unsuitable for the practice of this invention. Even if Me is replaced by Ti, Sn or Zr, the same trend will also be observed. Where it is desired to obtain a varistor having a relatively great voltage nonlinear coefficient,

it is preferred that the content of Me'O be in the range of 60 to 30 mol and the content of ZnO be in the range of 35 to 57 mol When it is desired to obtain a varistor whose rising voltage is relatively low, it is preferred that the content of Me'O be 14 to mol or around 70 mol and the content of ZnO be 66 to 85 mol or around mol An oxide varistor according to this invention contains, as an additive to the base Me'O -ZnO-Sb O composition, 1 to 20 weight of Bi O and 0.5 to 10 weight of Me" O The Me" O is selected from Fe O Cr O Mn O- C0 0 or a mixture thereof. Determination was made of the voltage nonlinear coefficient a of resistors obtained by adding varying amounts of Bi O to a basic composition formed of 30 mol of Me'O 60 mol of ZnO and 10 mol of Sb O in which Me' O is used as a parameter. The results are shown in FIGS. 3A to 6C. Figures with the suffix a added show the case where Me' is Ti; figures with the suffix [2 added show the case where Me is Sn; and figures with the suffix 0 added show the case where Me' is Zr. FIGS. 3A to 3C show the case where Me" is Fe; FIGS. 4A to 4C where Me" is Cr; FIGS. 5A to SC where Me" is Mn; and FIGS. 6A to 6C where Me" is Co.

As will be understood from FIGS. 3A to 6C, when the contents of Bi O and Me" O are ouside the scope indicated, a variator whose voltage nonlinear coefficient is great (a 30) is not obtained. Furthermore, those outside the scope have the drawback that a high rise voltage is involved. Even when a composition is outside the scope, there may be obtained a varistor whose voltage nonlinear coefficient exceeds 30. However, the varistor is higher (about 1.5 to 2 times higher) in its rise voltage than a variator according to this invention which shows a voltage nonlinear coefficient of much the same magnitude. Therefore, it is difficult to handle as well as to be put to practical use.

The above-mentioned voltage-current characteristics are not varied in any composition of this invention even when Ag or an In-Ga alloy is used as electrode material.

An oxide varistor according to this invention has as high a voltage nonlinear coefficient 01 as more than 30 and presents extremely small changes with temperature in the rising voltage and a high surge resistance, thus attaining a high performance. Therefore, the varistor is suitably applicable to an arrester, a surge suppressor for vacuum breaker, etc., as well as to the protection of communication instruments against surge and the suppression of abnormal voltage involved in a microwave oven.

Furthermore, an oxide varistor according to this invention can be manufactured at lower cost, since the raw material is obtained at low cost.

This invention will be more fully understood upon reading the following examples.

EXAMPLES Mixtures were prepared as shown in tables by adding to a basic composition formed of to 9 mol of Me'O2, 24 to mol of ZnO and l to 22 mol of Sb O (totalling mol 0.5 to 25 wt of Bi O and 0.3 to 12 wt of Me" O based on the basic composition, and intimately mixed at a ball mill. The mixtures were preliminarily sintered at 800C for 1 hour and powdered at a ball mill. Then, the powders were mixed with polyvinyl alcohol acting as a binder and shaped under a pressure of 1,000 Kg/cm The shaped powders were heated to a temperature ranging l,l00 to l,300C and kept at that temperature for 2 hours for sintering, thereby obtaining 147 discs having a diameter of 20 mm and a thickness of 1 mm. Then, silver electrodes were baked to the disc, resulting in a varistor. An elemental Ag or Ag O may likewise be used as a starting material for silver electrodes. Since the sintered mass is stable to temperature, the electrode could be baked to the sintered disc over a wider range of about 400 to 800C. Then, determination was made of the voltage-current characteristics of the samples, i.e., a varistor voltage Vc at room temperature and a voltage nonlinear coefficient a using a standard method. The results are shown in Tables I to III. Table I shows the case where Me' is Ti; Table II where Me is Sn; and Table III where Me'is Zr.

Table, lll Cntinued Main component Supplemental Sample No. (mol 71) component (wt 7:) Vc a 2:0 ZnO sb o, 131 0,, Mc'go. (v) Control 124 30 57 13 12 '657 35.8 125 Me=Fc 6.0 361 79.6 126 Mc=Cr 350 77.7 127 Me=Mn 344 76.7 128 Me=Co 365 80.2 Control 129 20 64 l6 I4 545 25.0 130 M'c=Fc 8.0 282 61.5 131 Mc=Cr 271 60.3 132 Me=Mn 264 59.8 133 Mc=Co 287 62.4 134 Me=Fe 2.0 290 62.7

Me=Cr Mc=Mn Me=Co Control 135 14 66 20 20 309 111 I36 Me=Fe 10.0 173 38.0 137 Me=Cr I61 369 138 Me=Mr1 155 36.2 139 Me==Co 182 39.3 Control 140 14 85 1 20 290 10.5 141 Me=Fc 0.5 130 33.0 143 Mc=Cr 122 32.4 143 Me=Mn 118 31.6 144 Me=Co 137 34.5 Control [45 75 24 l .25 Mc=Fe .3 291 16.6 Control 146 9 90 l (1.5 Mc=Cr Pl) 256 13.7 Control 147 48 3O 22 6 Mc=Mn 4.0 320 20.3

As will be a reciated from Tables I to III, the varis- Table IV Continued PP tor having a basic composition formed of 60 to 30 mol of Me'O 35 to 57 mol of ZnO and to mol I N ng t Surge Volltage of Sb- O exhibits an extremely high voltage nonlin- 3() Samp e f zfi g 'gg mlstmce lgg ear coefficient. Furthermore. the varistor having a voltage y (A/ 2) a basic composition formed of 14 to mol or about v 70 mol Of Me'O- 64 to 85 mol or about mol 7;; 2 4100 g 1 of ZnO and l to 20 mol of Sb O has a particularly 2: low rise volmge- 93 0:00i 34 20 324 With respect to some of the above-mentioned exam- 100 0.003 3240 35.1

7 7 ples determmation was made of the change with tem- :8; 3:: perature in the rising voltage (Vc) and surge resistance 113 ()()2 4 8, involved in a pulse current of 8 X 20 us. The results are i 2328 shown in Table IV in which the voltage nonlinear coef- 5 h 4440 0}, ficient a as shown in Tables I to II] is restated. Table IV 139 0:001 3450 35:3

Chan 0 with Surge Voltage Sample No. r s mrc resistance u As w1ll be understood from Table IV, the change w1th m ?;'F% (A/ 2) g temperature in the rising voltage of the oxide varistor VO e 0 cm g according to th1s 1nvent1on 1s below 0.003 %/C and Control 1 0.004 2570 11.0 the varistor is very favorably compared with the cong lventional SiC varistor (0.l %/C) and zener diode 48 2130 (0.1 %/C). Furthermore, the surge resistance of the 50 2550 1 1.2 varistor according to th1s 1nvent1on 1s more than 3,000 N I888: 3:28 A/cm and very favorably compared with the conven- 97 0.009 2220 13.7 tional zinc oxide varistor element (2,000 A/cm and zener diode (20 A/cm H 139 41004 5 With the varistors outside the scope of this invention, 2 gig for example, the sample Nos. 13, 26, 62, 75, l l 1, etc., 2, 8 1 the voltage nonlinear coefficient a exceeds 30 and they 9 0.002 3970 72.5 are favorably compared with some of the varistors acm O'003 4220 cordin to this invention. However, the are inferior, in 21 0.001 4630 110.0 g Y 24 4950 116,; the change w1th temperature in the rising voltage as 38 well as in the surge resistance, to the varistors accord- 34 0.003 3870 58.2

38- 0.002 3640 36.0 mg to t mvemlon- 43 0.001 3380 2.7 The varlstor accordlng to th1s lnventlon was supplled 53 22 i will an electric power of 1 watt for a load life test and 50 0.001 3930 75,3 kept at C for 500 hours. Therefore, the variation of ff 22: 8 the voltage nonlinear coefficient a was determined. 74 h 4950 1 The results are shown below in comparison with the resistors of US. Pat. No. 3,663,458 which are described in Table 12 of this patent.

Table V Variation of voltage nonlinear From this table it will be understood that the varistor according to this invention undergoes a very small change with time. Though Me' is used in a single compound, the same result is obtained even when it is used in a mixed form.

What we claim is:

1. An oxide varistor prepared from a basic composition consisting of a total of 70 to 14 mol of at least one compound of the formula Me'O 29 to 85 mol of ZnO and l to 20 mol of sb o and containing 1 to 20 by weight of Bi O and a total of 0.5 to by weight of at least one Me" O based on the weight 12 of said basic composition, wherein said Me is selected from the group consisting of Ti, Sn and Zr, said Me" is selected from the group consisting of Fe, Cr, Mn and Co.

2. The oxide varistor according to claim 1 wherein said basic composition consists of 60 to 30 mol of Me'O 35 to 57 mol of ZnO and 5 to 15 mol of Sb O 3. The oxide varistor according to claim 1, wherein the amount of said Me'0 of the basic composition is 20 to 14 mol and the amount of said ZnO of the basic composition is 66 to 85 mol 4. The oxide varistor according to claim 1, wherein the amount of said Me'0 of the basic composition is about 70 mol and the amount of said ZnO of the basic composition is about 30 mol 5. An oxide varistor having a nonlinear voltage coefficient (a) greater than 30, a change with temperature in rising voltage of less than 0.003%/C and a surge resistance of more than 3,000 A/cm said oxide varistor being prepared from a basic composition consisting of a total of 70 to 14 mol of at least one compound of the formula Me'O 29 to mol of ZnO and l to 20 mol of Sb O and containing 1 to 20 by weight of Bi O and a total of 0.5 to 10 by weight of at least one Me" O based on the weight of said basic composition, wherein said Me is selected from the group consisting of Ti, Sn and Zr, and said Me" is selected from the group consisting of Fe, Cr, Mn and Co. 

1. AN AXIDE VARISTOR PREPARED FROM A BASIC COMPOSITION CONSISTING OF A TOTAL OF 70 TO 14 MOL % OF AT LEAST ONE COMPOUND OF THE FORMULA MCIO2, 29 TO 85 MOL % OF ZNO AND 1 TO 20 MOL % OF SB2O3, AND CONTAINING 1 TO 20 % BY WEIGHT OF BI2O3, AND A TOTAL OF 0.5 TO 10 % BY WEIGHT OF AT LEAST ONE MEII2O3 BASED ON THE WEIGHT OF SAID BASIC COMPOSITION, WHEREIN SAID MEI IS SELECTED FROM THE GROUP CONSISTING OF TI, SN AND ZR, SAID MEII IS SELECTED FROM THE GROUP CONSISTING OF FE, CR, MN AND CO.
 2. The oxide varistor according to claim 1 wherein said basic composition consists of 60 to 30 mol % of MeIO2, 35 to 57 mol % of ZnO and 5 to 15 mol % of Sb2O3.
 3. The oxide varistor according to claim 1, wherein the amount of said MeIO2 of the basic composition is 20 to 14 mol % and the amount of said ZnO of the basic composition is 66 to 85 mol %.
 4. The oxide varistor according to claim 1, wherein the amount of said MeIO2 of the basic composition is about 70 mol % and the amount of said ZnO of the basic composition is about 30 mol %.
 5. An oxide varistor having a nonlinear voltage coefficient ( Alpha ) greater than 30, a change with temperature in rising voltage of less than -0.003%/*C and a surge resistance of more than 3,000 A/cm2, said oxide varistor being prepared from a basic composition consisting of a total of 70 to 14 mol % of at least one compound of the formula MeIO2, 29 to 85 mol % of ZnO and 1 to 20 mol % of Sb2O3, and containing 1 to 20 % by weight of Bi2O3, and a total of 0.5 to 10 % by weight of at least one MeII2O3 based on the weight of said basic composition, wherein said MeI is selected from the group consisting of Ti, Sn and Zr, and said MeII is selected from the group consisting of Fe, Cr, Mn and Co. 